The present invention relates to a micromechanical structure, e.g., for an acceleration sensor or rotational speed sensor, including a substrate, which includes an anchoring device and a centrifugal mass, which is connected to the anchoring device via a flexible spring device, so that the centrifugal mass is elastically deflectable from its rest position. The present invention also provides a manufacturing method for said micromechanical structure.
Although it is usable on any micromechanical structure, the present invention and the problem on which it is based will be described in relation to a micromechanical rocker structure for an acceleration sensor or rotational speed sensor.
FIG. 6 shows a schematic top view of a known micromechanical structure in the form of a rocker structure for an acceleration sensor or rotational speed sensor; FIG. 6a shows the micromechanical structure in top view and FIG. 6b in a sectional view along line C-Cxe2x80x2.
Micromechanical acceleration sensors and rotational speed sensors are conventional. The operation and configuration of an acceleration sensor are based on elastic vertical sensitivity, i.e., a direction of detection, to acceleration which is perpendicular to the chip plane.
A conventional micromechanical structure includes a substrate, which includes an anchoring device and a centrifugal mass in the form of a rocker including longitudinal and transverse bars and corresponding clearances, which is connected to anchoring device via a flexible spring device, so that centrifugal mass is elastically deflectable from its rest position.
This deflectability is implementable by etching a sacrificial layer 50 under centrifugal mass 30. Sacrificial layer 50 is composed of a lower sacrificial sublayer 51 and an upper sacrificial sublayer 52, between which electrode areas 60 are provided, which cooperate electrostatically with centrifugal mass 30.
In this structure, sacrificial layer 50 is present in a first area situated under centrifugal mass 30 with a first etchable thickness d1, and in a second area situated under centrifugal mass 30 with a second etchable thickness d1+d2+d3, second thickness d1+d2+d3 is greater than first thickness d1.
Thus, in this component, the movable components are situated in an upper electromechanically functional plane and are made of epitactical polysilicon. Under this plane, at a distance corresponding to sacrificial sublayer thickness d1, is a second electrically functional plane made of doped silicon, which acts as a capacitive counterelectrode to the upper functional layer.
This basic layer structure of such a vertically sensitive acceleration sensor is shown along line C-Cxe2x80x2 in FIG. 6b. Sacrificial layer 50 has been selectively removed according to FIG. 6b. Sacrificial sublayer 52 has been left in place in the area of the anchoring device in order to connect the latter to underlying layers 60, 51, and thus to substrate 10.
In order to manufacture this structure, sacrificial sublayer 51 including a layer thickness d3 is deposited on underlying substrate 10 using a CVD method. The electrode layer made of doped silicon and including layer thickness d2 is deposited on sacrificial sublayer 51 and structured to form electrode areas 60. Subsequently sacrificial sublayer 52 including layer thickness d1 is deposited on underlying electrode areas 60, i.e., sacrificial sublayer 51 using the CVD method. Finally, centrifugal mass 30 is formed from an epitactical polysilicon layer, and sacrificial layer 50 is etched to provide deflectability.
When etching sacrificial layer 50, i.e., sacrificial sublayers 51, 52, electrode areas 60 are not attacked, so that they act as etching depth stops. In contrast, etching between electrode areas 60 proceeds to substrate 10.
The layer thickness of CVD sacrificial sublayers 51, 52 is usually between 2.0 xcexcm and 1.0 xcexcm. They are usually made of TEOS (tetraethoxysilane) oxide, or of silane oxide.
TEOS is obtained via the following chemical reaction:
Si(OC2H5)4xe2x86x92SiO2+organic reaction products.
Silane oxide is obtained via the following reaction:
SiH4+4N2Oxe2x86x92SiO2+4N2+2H2O
FIGS. 7a-7c show enlarged details of the micromechanical structure according to FIG. 6 underneath the movable bars during different phases of the sacrificial layer etching process.
The CVD deposition technique of sacrificial sublayers 51, 52 causes contaminants 70 from the reaction products, such as for example organic components, to be incorporated in TEOS (schematically represented in FIG. 7 for sacrificial sublayer 52) or nitrogen inclusions to be incorporated in silane oxide.
This undesirable incorporation only slightly impairs the electrical insulating properties of sacrificial sublayers 51, 52, so that the microelectromechanical applications are unaffected.
However, contaminants 70 may cause a problem when they are not attacked during selective etching of sacrificial layer 50 or when they react with the etching medium forming non-soluble and/or non-volatile compounds as residues. During sacrificial layer etching, these residues become enriched from the already etched portions of the oxide on the SiO2 etching front (see FIG. 7b). They may cross-link, agglomerate to form larger structures, and remain on electrode areas 60 underneath movable centrifugal mass 30 as solid, non-conductive particles having a size of up to layer thickness d1 of sacrificial sublayer 51 (FIG. 7c). This creates the danger of these particles blocking the deflection of centrifugal mass 30 in the z direction or of producing an electrical short circuit therewith.
Intensive research has yielded the surprising finding that, duexe2x80x2 to the enrichment mechanism on the etching front (FIGS. 7a-7c), larger residue particles are formed in critical areas where during sacrificial layer etching multiple etching fronts come together shortly before the process end. In this case, only small particles having a small height, which are not critical for the functionality of z-sensitive centrifugal masses 30, are formed when two etching fronts come together.
However, if three or more etching fronts come together, high particle structures may form, which greatly reduce the mechanical functional area.
FIGS. 8a-8c schematically show critical areas of the micromechanical structure according to FIGS. 6 and 7.
Critical points in the micromechanical structure, i.e., the sensor configuration, where more than two etching fronts come together, are located
a) under end 36 of underetched, free-standing bar structures 35 (FIG. 8a),
b) under flexion points 37 of underetched, free-standing bar structures 35 (FIG. 8b),
c) under points of intersection 38 of underetched, free-standing bar structures 35, as formed, for example, due to holes or clearances 39 for better underetching (FIG. 8c).
For vertically sensitive acceleration sensors and rotational speed sensors, the above-named structure elements a) to c) are used to configure the mechanically functional plane. In the case of large-surface, horizontally arranged capacitance electrodes in the functional layer, the holes are arranged close together on the surface in order to achieve sufficient underetching. Previously, attention was only paid to optimum underetching or free etching of the mechanical structure, but not to possible etching residues.
The formation of etching residues at the above-mentioned points in the sensor configuration represents a problem in z-sensitive components in which sacrificial layer 50 is removed in a dry, isotropic etching step. Here they may not be detached or rinsed away by a liquid phase of the etching medium. They are critical for the functionality if they are located underneath the electromechanically functional structures such as, for example, the rocker structure of a z acceleration sensor, and their size approximately corresponds to the distance between the electrically functional layer and the electromechanically functionally layer above it.
It is an object of the present invention to eliminate etching residues that may impair the function of micromechanical components and structures.
The present invention provides that the centrifugal mass has a certain structure with clearances, which may be made deflectable by etching a sacrificial layer underneath it. The sacrificial layer is present in a first area located underneath the centrifugal mass with a first etchable thickness and in a second area located underneath the centrifugal mass with a second etchable thickness, the second thickness is greater than the first thickness. The centrifugal mass is structured in the first area so that in etching only a maximum of two etching fronts may come together in order to limit etching residue deposits.
The present invention may provide the advantage that it provides an effective configuration for micromechanical components or structures to avoid mechanically or electrically interfering influences of etching residues.
Using the present invention, the formation of etching residues which are critical in terms of their size under electromechanically functional micromechanical structures is virtually eliminated. Thus it is achieved that the process yield of freely vibrating sensor structure is increased and a possible safety risk due to the failure of blocked z-sensitive sensors, for example, if used in motor vehicles, is reduced.
Example embodiments of the present invention are illustrated in the drawings and described in greater detail in the description that follows.